Phytoextraction of heavy metals from contaminated soil by co-cropping with chelator application and assessment of associated leaching risk

Field experiments to assess the remediation efficiency of metal-contaminated soil by flushing with ferric chloride followed by applying amendments

The Cd, Cu, Pb, and Zn removal efficiencies achieved by flushing with FeCl₃ were determined in a field experiment using soil contaminated with multiple metals. Soil was first flushed with FeCl₃ and then with FeCl₃ or a mixture of chelators. Flushed soil was amended with lime and organic matter to revitalize the soil, then the soil was used to grow Zea mays and Brassica juncea. The heavy metal concentrations in groundwater were determined to assess the risks of leaching caused by soil flushing. The Cd, Cu, Pb, and Zn removal efficiencies were 70%, 40%, 33%, and 17%, respectively, when FeCl₃ (25 mmol (kg topsoil)^-1) was applied. The second washing generally did not significantly decrease the heavy metal contents of the soil but the second FeCl₃ washing did decrease the Pb content. Pb leached from topsoil was partly retained by the subsoil 20-40 cm deep. The Zea mays yields were significantly lower but the Brassica juncea yields were significantly higher after the combined soil flushing and amendment treatment than after only the amendment treatment. This indicated that soil flushing only negatively affected growth of deep-rooted Z. mays. The Cd, Cu, Pb, and Zn concentrations in Z. mays grains and the edible parts of B. juncea grown in remediated soil were below the Chinese tolerable limits for contaminants in food. Washing with FeCl₃ did not increase groundwater contamination during the study. The results indicated that flushing soil with FeCl₃ and subsequent amendments is a technically feasible method for remediating agricultural soil contaminated with Cd.

Phytoextraction of Zn and Cd with Arabidopsis halleri: a focus on fertilization and biological amendment as a means of increasing biomass and Cd and Zn concentrations

The current work aims to investigate the influence of fertilization (fertilizer) and fungal inoculation (Funneliformis mosseae and Serendipita indica (formerly Piriformospora indica), respectively arbuscular mycorrhizal (AMF) and endophytic fungi) on the phytoextraction potential of Arabidopsis halleri (L.) O'Kane & Al-Shehbaz (biomass yield and/or aboveground part Zn and Cd concentrations) over one life plant cycle. The mycorrhizal rates of A. halleri were measured in situ while the fungal inoculation experiments were carried out under controlled conditions. For the first time, it is demonstrated that the fertilizer used on A. halleri increased its biomass not only at the rosette stage but also at the flowering and fruiting stages. Fertilizer reduced the Zn concentration variability between developmental stages and increased the Cd concentration at fruiting stage. A. halleri roots did not show AMF colonization at any stage in our field conditions, neither in the absence nor in the presence of fertilizer, thus suggesting that A. halleri is not naturally mycorrhizal. Induced mycorrhization agreed with this result. However, S. indica has been shown to successfully colonize A. halleri roots under controlled conditions. This study confirms the benefit of using fertilizer to increase the phytoextraction potential of A. halleri. Overall, these results contribute to the future applicability of A. halleri in a phytomanagement strategy by giving information on its cultural itinerary.

Rhodococcus qingshengii facilitates the phytoextraction of Zn, Cd, Ni, and Pb from soils by Sedum alfredii Hance

The enhanced heavy metal (HM) phytoextraction efficiency of hyperaccumulating plants via plant-growth-promoting microbes has been proposed as an effective strategy to remove HMs from contaminated soil. Nevertheless, it remains unclear whether catabolizing the abscisic acid (ABA) in hyperaccumulating plants via rhizobacteria can facilitate HM phytoextraction. In the present study, a hyperaccumulator, Sedum alfredii Hance, inoculated with an ABA-catabolizing bacterium Rhodococcus qingshengii, showed higher concentrations of Zn, Cd, Ni, and Pb in the contaminated paddy-grown plant shoots by 35%, 63%, 49%, and 49%, and in plants grown in mine soils by 112%, 105%, 46%, and 49%, respectively, than in the control_{bacteria-free} plants. However, no significant changes were observed in Cu content between these plants. Furthermore, parameters indicating phytoremediation potential, including the translocation factor (TF) and bioconcentration factor (BCF), revealed that bacterial inoculation could markedly increase the efficacy of Zn, Cd, Ni, and Pb phytoextraction from the soil. Notably, the bioavailabilities of HMs in soils were not influenced by R. qingshengii; however, the expression of transporters related to the uptake of these HMs, including SaIRT1, SaZIP1, SaZIP2, SaZIP3, SaNramp1, SaNramp3, SaNramp6, SaHMA2, and SaHMA3, was upregulated. These findings indicate that R. qingshengii inoculation could increase the HM-uptake ability of plants by catabolizing ABA and may provide a promising strategy for enhancing the phytoremediation efficacy in HM-contaminated soils.

Effect of different direct revegetation strategies on the mobility of heavy metals in artificial zinc smelting waste slag: Implications for phytoremediation

The establishment of vegetation cover is an important strategy to reduce wind and water erosion at metal smelting waste slag sites. However, the mobility of heavy metals in waste slag-vegetation-leachate systems after the application of revegetation strategies is still unclear. Large microcosm experiments were conducted for revegetation of waste slag for 98 d using combined amendments, i.e., phosphate rock and an organic waste coming from the anaerobic digestion of pig manure (named as biogas residue), and by single- and co-planted perennial ryegrass (Lolium perenne L.) and Trifolium repens (T. repens). The results showed that the application of biogas residue slightly increased the concentrations of Zn and Cd in the leachates; however, the establishment of plants could avoid the excessive leaching of heavy metals coming from the biogas residue. The bioavailability of Cu, Zn, and Cd slightly increased, but Pb bioavailability significantly decreased regardless of single- or co-planting patterns. Additionally, the bioavailability of Cu, Zn, and Cd in the waste slag revegetated with perennial ryegrass was lower than that in T. repens under the single-planting pattern. The change in the heavy metals bioavailability under different revegetation strategies was mainly due to the root-induced change in the pH and speciation of heavy metals in the waste slag. The application of biogas residue and phosphate rock tends to the immobilization of Pb. Heavy metals mainly accumulated in the underground parts of the two herbs, and the heavy metal contents in the underground parts of perennial ryegrass were higher than those in T. repens regardless of single- or co-planting patterns. The heavy metals accumulated in T. repens were lower than those in perennial ryegrass in the single-planting pattern. The bioaccumulation and transportation factors of the two herbs were extremely low. Thus, the two herbs are potential candidates for phytostabilization of zinc smelting waste slag sites.

Soil Threshold Values for Zn Based on Soil-Rice System and Health Risk Assessment in a Typical Carbonate Area of Guangxi

Zinc (Zn) is enriched in carbonate area related to geological genesis. To ensure safety of rice, soil threshold values of Zn in soil-rice systems were assessed based on analysis of soil-rice Zn concentration in relation to human health risk. Models for the prediction of Zn concentration of early-season and late-season rice grain were accurately established on the basis of significant partial correlations between log₁₀ (BAFs) and log₁₀ (soil properties). The rice threshold value ranged from 10.67 to 37.90 mg/kg, which might not suitable for male and urban residents. The soil safety threshold of early-season rice and late-season rice in carbonate area ranged from 148-200 mg/kg, 119-200 mg/kg with pH below 6.5, 148-250 mg/kg, 119-250 mg/kg with pH ranging from 6.5 to 7.5; 148-300 mg/kg, 119-300 mg/kg with pH above 7.5, respectively.

Using bioenergy crop cassava (Manihot esculenta) for reclamation of heavily metal-contaminated land

Heavy metal contamination of agricultural lands may give rise to health risks by cultivation and consumption of food crops from such lands, as well as result in economic loss. Phytoremediation is an eco-friendly and cost-effective approach to restore contaminated soil. However, the restoration process is slow and its sustainability is difficult to maintain. Bioenergy crops may provide alternative economic benefits to agriculture sector and reduce the risks associated with transfering heavy metals into food webs. In this study, a field experiment was carried out to determine the level of reclamation that would be attained in severely heavy metal-contaminated land by planting cassava (Manihot esculenta), a bioenergy crop. The results showed that cassava could grow well on the derelict land, with a fresh tuber yield of 23.13-26.22 t ha^-1 in one growing season, which could potentially produce 3680-4160 L ha^-1 bioethanol. The economic income of the cassava was estimated to be 11.6-13.1 × 10³ CNY ha^-1. Among the cassava tissues, metal concentrations were lowest in the tuber. The soil fertility and acidity were ameliorated after cassava plantation, and the mobile and bioavailable metal fractions in the soils were decreased. The cultivation of cassava as a renewable energy crop appears applicable for sustainable utilization and reclamation of heavy metal-contaminated land.

Glutamic Acid-Assisted Phytomanagement of Chromium Contaminated Soil by Sunflower (Helianthus annuus L.): Morphophysiological and Biochemical Alterations

Chelator-assisted phytoremediation is an economical, sustainable, and ecologically friendly method of extracting heavy metals and metalloids from the soil. Organic chelators are thought to enhance metal availability and mobility in contaminated media, thereby improving phytoextraction. The aim of the present study was to examine whether exogenous application of glutamic acid (GA) could improve chromium (Cr) phytoextraction by sunflower plants (Helianthus annuus L.). Seeds were planted in plastic pots filled with 5 kg of local agricultural soil spiked with increasing concentrations of Cr (1, 2, and 5 mg kg^-1). Glutamic acid (5 mM) was applied to soil in solution according to a completely randomized experimental design, and the sunflower plants were harvested after 8 weeks. The results indicated that increasing Cr-induced stress significantly inhibited plant growth, leading to reduced biomass, photosynthetic pigment content, activities of antioxidant enzymes, and leaf area of the sunflower plants. However, exogenous addition of GA significantly reduced the Cr-associated toxic effects while also increasing the accumulation of Cr in the plants. Moreover, increasing concentrations of Cr in the soil increased the generation of reactive oxygen species (ROS) responsible for the altered antioxidant enzyme activities. The results revealed that GA application to the topsoil enhanced the Cr concentration and accumulation in the root, stem, and leaves by up to 254, 225, 355, and 47, 59, 150% respectively. Further the GA addition reduced the Cr-induced toxicity in plants and might be helpful for enhancing Cr phytoextraction by sunflower plants.

Combining potassium chloride leaching with vertical electrokinetics to remediate cadmium-contaminated soils

This study evaluated the feasibility of combining potassium chloride (KCl) leaching and electrokinetic (EK) treatment for the remediation of cadmium (Cd) and other metals from contaminated soils. KCl leaching was compared at three concentrations (0.2%, 0.5%, and 1% KCl). EK treatment was conducted separately to migrate the metals in the topsoil to the subsoil. The combined approach using KCl leaching before or after EK treatment was compared. For the single vertical EK treatment, the removal of Cd, lead (Pb), copper (Cu) and zinc (Zn) from the topsoil (0-20 cm) was 9.38%, 4.80%, 0.95%, and 10.81%, respectively. KCl leaching at 1% KCl removed 84.06% Cd, 9.95% Pb, 4.34% Cu, and 19.93% Zn from the topsoil, with higher removal efficiency than that of the 0.2% and 0.5% KCl leaching treatments. By combining the KCl leaching and EK treatment, the removal efficiency of heavy metals improved, in particular for the 1% KCl + EK treatment, where the removal rate of Cd, Pb, Cu, and Zn from the upper surface soil reached 97.79%, 17.69%, 14.37%, and 41.96%, respectively. Correspondingly, the soil Cd content decreased from 4 to 0.21 mg/kg, and was below the Chinese standard limit of 0.3 mg/kg soil. These results indicate that 1% KCl + EK treatment is a good combination technique to mitigate Cd pollution from contaminated soils used for growing rice and leafy vegetables.

Effects of mutual intercropping on cadmium accumulation by the accumulator plants Conyza canadensis, Cardamine hirsuta, and Cerastium glomeratum

In this study, three cadmium (Cd) accumulator species (Conyza canadensis, Cardamine hirsuta, and Cerastium glomeratum) were co-cultured in Cd-contaminated soil in pots to study the effects of intercropping on co-remediation. Only C. canadensis intercropped with C. glomeratum, C. hirsuta intercropped with C. glomeratum, and three-species intercropping increased plant biomass compared with their respective monocultures. The treatments of C. canadensis intercropped with C. glomeratum and three-species intercropping increased the Cd contents in roots and shoots of C. canadensis, whereas the other intercropping treatments decreased or had no significant impact on Cd contents. As for Cd accumulation, the treatments of C. canadensis intercropped with C. glomeratum, C. hirsuta intercropped with C. glomeratum, and three-species intercropping increased Cd accumulation in a single plant compared with that of their respective monocultures, whereas other intercropping treatments decreased Cd accumulation in individual plants. Only the treatments of C. canadensis intercropped with C. glomeratum and C. hirsuta intercropped with C. glomeratum increased Cd accumulation in shoots of a single pot compared with that of their respective monocultures. Therefore, C. canadensis intercropped with C. glomeratum and C. hirsuta intercropped with C. glomeratum may improve the phytoremediation efficiency for Cd-contaminated soil.

Enhanced phytoremediation of cadmium and/or benzo(a)pyrene contaminated soil by hyperaccumlator Solanum nigrum L

The role of same amendment on phytoremediating different level contaminated soils is seldom known. Soil pot culture experiment was used to compare the strengthening roles of cysteine (CY), EDTA, salicylic acid (Sa), and Tween 80 (TW) on hyperaccumulator Solanum nigrum L. phytoremediating higher level of single cadmium (Cd) or Benzo(a)pyrene (BAP) and their co-contaminated soils. Results showed that the Cd capacities (ug pot^-1) in shoots of S. nigrum in the combined treatment T_{0.1EDTA+0.9CY} were the highest for the 5 and 15 mg kg^-1 Cd contaminated soils. When S. nigrum remediating co-contaminated soils with higher levels of Cd and BAP, that is, 5 mg kg^-1 Cd + 1 mg kg^-1 BAP and 15 mg kg^-1 Cd + 2 mg kg^-1 BAP, the treatment T_{0.9CY+0.9Sa+0.3TW} showed the best enhancing remediation role. This results were different with co-contaminated soil with 0.771 mg kg^-1 Cd + 0.024 mg kg^-1 BAP. These results may tell us that the combine used of CY, SA, and TW were more useful for the contaminated soils with higher level of Cd and/or BAP. In the combined treatments of Sa+TW, CY was better than EDTA.

Leaching variations of heavy metals in chelator-assisted phytoextraction by Zea mays L. exposed to acid rainfall

Chelant-enhanced phytoextraction method has been put forward as an effective soil remediation method, whereas the heavy metal leaching could not be ignored. In this study, a cropping-leaching experiment, using soil columns, was applied to study the metal leaching variations during assisted phytoextraction of Cd- and Pb-polluted soils, using seedlings of Zea mays, applying three different chelators (EDTA, EDDS, and rhamnolipid), and artificial rainfall (acid rainfall or normal rainfall). It showed that artificial rainfall, especially artificial acid rain, after chelator application led to the increase of heavy metals in the leaching solution. EDTA increased both Cd and Pb concentrations in the leaching solution, obviously, whereas EDDS and rhamnolipid increased Cd concentration but not Pb. The amount of Cd and Pb decreased as the leaching solution increased, the patterns as well matched LRMs (linear regression models), with R-square (R ²) higher than 90 and 82% for Cd and Pb, respectively. The maximum cumulative Cd and Pb in the leaching solutions were 18.44 and 16.68%, respectively, which was amended by EDTA and acid rainwater (pH 4.5), and followed by EDDS (pH 4.5), EDDS (pH 6.5), rhamnolipid (0.5 g kg^-1 soil, pH 4.5), and rhamnolipid (pH 6.5).

Leaching characteristics of EDTA-enhanced phytoextraction of Cd and Pb by Zea mays L. in different particle-size fractions of soil aggregates exposed to artificial rain

Chelator-assisted phytoextraction is an alternative and effective technique for the remediation of heavy metal-contaminated soils, but the potential for heavy metal leaching needs to be assessed. In the present study, a soil column cultivation-leaching experiment was conducted to investigate the Cd and Pb leaching characteristics during assisted phytoextraction of metal-contaminated soils containing different particle-size soil aggregates. The columns were planted with Zea mays "Zhengdan 958" seedlings and treated with combined applications of EDTA and simulated rainfall (pH 4.5 or 6.5). The results were as follows: (1) The greatest uptake of Cd and Pb by Z. mays was observed after treatment with EDTA (2.5 mmol kg^-1 soil) and soil aggregates of <1 mm; uptake decreased as the soil aggregate size increased. (2) Simulated rainfall, especially acid rain (pH 4.5), after EDTA applications led to the increasing metal concentrations in the leachate, and EDTA significantly increased the concentrations of both Cd and Pb in the leachate, especially with soil aggregates of <1 mm; metal leachate concentrations decreased as soil particle sizes increased. (3) Concentrations of Cd and Pb decreased with each continuing leachate collection, and data were fit to linear regression models with coefficients of determination (R ²) above 0.90 and 0.87 for Cd and Pb, respectively. The highest total amounts of Cd (22.12%) and Pb (19.29%) were observed in the leachate of soils treated with EDTA and artificial acid rain (pH 4.5) with soil aggregates of <1 mm. The application of EDTA during phytoextraction method increased the leaching risk in the following order: EDTA_2.5-1 (pH 4.5) > EDTA_2.5-1 (pH 6.5) > EDTA_2.5-2 (pH 4.5) > EDTA_2.5-4 (pH 4.5) > EDTA_2.5-2 (pH 6.5) > EDTA_2.5-4 (pH 6.5).

Effects of co-cropping Bidens pilosa (L.) and Tagetes minuta (L.) on bioaccumulation of Pb in Lactuca sativa (L.) growing in polluted agricultural soils

Polluted agricultural soils are a serious problem for food safety, with phytoremediation being the most favorable alternative from the environmental perspective. However, this methodology is generally time-consuming and requires the cessation of agriculture. Therefore, the purpose of this study was to evaluate two potential phytoextractor plants (the native species Bidens pilosa and Tagetes minuta) co-cropped with lettuce growing on agricultural lead-polluted soils. The concentrations of Pb, as well as of other metals, were investigated in the phytoextractors, crop species, and in soils, with the potential risk to the health of consumers being estimated. The soil parameters pH, EC, organic matter percentage and bioavailable lead showed a direct relationship with the accumulation of Pb in roots. In addition, the concentration of Pb in roots of native species was closely related to Fe (B. pilosa, r = 0.81; T. minuta r = 0.75), Cu (T. minuta, r = 0.93), Mn (B. pilosa, r = 0.89) and Zn (B. pilosa, r = 0.91; T. minuta, r = 0.91). Our results indicate that the interaction between rhizospheres increased the phytoextraction of lead, which was accompanied by an increase in the biomass of the phytoextractor species. However, the consumption of lettuce still revealed a toxicological risk from Pb in all treatments.

Effect of soil washing with only chelators or combining with ferric chloride on soil heavy metal removal and phytoavailability: Field experiments

In a field experiment on multi-metal contaminated soil, we investigated the efficiency of Cd, Pb, Zn, and Cu removal by only mixture of chelators (MC) or combining with FeCl3. After washing treatment, a co-cropping system was performed for heavy metals to be extracted by Sedum alfredii and to produce safe food from Zea mays. We analyzed the concentration of heavy metals in groundwater to evaluate the leashing risk of soil washing with FeCl3 and MC. Results showed that addition of FeCl3 was favorable to the removal of heavy metals in the topsoil. Metal leaching occurred mainly in rain season during the first co-cropping. The removal rates of Cd, Zn, Pb, and Cu in topsoil were 28%, 53%, 41%, and 21% with washing by FeCl3+MC after first harvest. The application of FeCl3 reduced the yield of S. alfredii and increased the metals concentration of Z. mays in first harvest. However, after amending soil, the metals concentration of Z. mays in FeCl3+MC treatment were similar to that only washing by MC. The grains and shoots of Z. mays were safe for use in feed production. Soil washing did not worsen groundwater contamination during the study period. But the concentration of Cd in groundwater was higher than the limit value of Standard concentrations for Groundwater IV. This study suggests that soil washing using FeCl3 and MC for the remediation of multi-metal contaminated soil is potential feasibility. However, the subsequent measure to improve the washed soil environment for planting crop is considered.

Long-term field phytoextraction of zinc/cadmium contaminated soil by Sedum plumbizincicola under different agronomic strategies

In two long-term field experiments the zinc (Zn)/cadmium (Cd) hyperaccumulator Sedum plumbizincicola (S. plumbizincicola) was examined to optimize the phytoextraction of metal contaminated soil by two agronomic strategies of intercropping with maize (Zea mays) and plant densities. Soil total Zn and Cd concentrations decreased markedly after long-term phytoextraction. But shoot biomass and Cd and Zn concentrations showed no significant difference with increasing remediation time. In the intercropping experiment the phytoremediation efficiency in the treatment "S. plumbizincicola intercropped with maize" was higher than in S. plumbizincicola monocropping, and Cd concentrations of corn were below the maximum national limit. In the plant density experiment the phytoremediation efficiency increased with increasing plant density and 440,000 plants ha(-1) gave the maximum rate. These results indicated that S. plumbizincicola at an appropriate planting density and intercropped with maize can achieve high remediation efficiency to contaminated soil without affecting the cereal crop productivity. This cropping system combines adequate agricultural production with soil heavy metal phytoextraction.

Co-Planting Cd Contaminated Field Using Hyperaccumulator Solanum Nigrum L. Through Interplant with Low Accumulation Welsh Onion

Monoculture and intercrop of hyperaccumulator Solanum nigrum L. with low accumulation Welsh onion Renbentieganchongwang were conducted. The results showed that the remove ratio of S. nigrum to Cd was about 7% in intercrop plot when top soil (0-20 cm) Cd concentration was 0.45-0.62 mg kg(-1), which did not significantly impact the yield of low accumulation Welsh onion compared to the monoculture. The consistency of remove ratio in practice and theory indicated the remediation of S. nigrum to Cd was significant. The Cd concentration and yield of Welsh onion were not affected by the growth of S. nigrum either in intercrop plot. The Cd concentration in edible parts of Welsh onion was available either. In short, inter-planting hyperaccumulator with low accumulation crop could normally remediate contaminated soil and produce crop (obtain economic benefit), which may be one practical pathway of phytoremediating heavy metal contaminated soil in the future.

Phytotreatment of sewage sludge contaminated by heavy metals and PAHs by co-planting Sedum alfredii and Alocasia marorrhiza

High concentrations of heavy metals and organic pollutants in municipal sewage sludge are key factors limiting its use in agriculture. The objectives of this study were to decrease the heavy metal and polycyclic aromatic hydrocarbon concentrations in sewage sludge by phytotreatment and to determine, in a field experiment, whether co-planting is more effective than using a mono-crop of Sedum alfredii. Four treatments were used in the plot experiment: no sludge, no plants, S. alfredii and co-planting S. alfredii and Alocasia marorrhiza. The results showed that co-planting produced tubers and shoots of A. marorrhiza that were suitable as a safe animal feed and good organic K fertilizer, respectively. Co-planting was more effective than mono-planting at reducing concentrations of total Zn and diethylenetriaminepentaacetic acid (DTPA)-extractable Zn, Cd, and Cu in the sludge. Co-planting decreased the concentrations of DTPA-extractable heavy metals and benzo[a]pyrene (B[a]P) in the sludge significantly compared with the unplanted sludge. Decreases of 87, 75, 85, 31, and 64% were obtained for B[a]P and DTPA-extractable Zn, Cd, Cu, and Pb, respectively, compared with the fresh sludge. These results indicate that co-planting can reduce significantly the environmental risks associated with heavy metals and B[a]P in sewage sludge for further disposal.

Moderate phosphorus application enhances Zn mobility and uptake in hyperaccumulator Sedum alfredii

While phytoextraction tools are increasingly applied to remediation of contaminated soils, strategies are needed to optimize plant uptake by improving soil conditions. Mineral nutrition affects plant growth and metal absorption and subsequently the accumulation of heavy metal through hyper-accumulator plants. Microcosm experiments were conducted in greenhouse to examine the effect of different phosphorus (P) sources on zinc (Zn) phytoextraction by Sedum alfredii in aged Zn-contaminated paddy soil. The Zn accumulation, soil pH, microbial biomass and enzyme activity, available Zn changes. and Zn phytoremediation efficiency in soil after plant harvest were determined. Upon addition of P, Zn uptake of S. alfredii significantly increased. Mehlich-3 extractable or the fractions of exchangeable and carbonate-bound soil Zn were significantly increased at higher P applications. Soil pH significantly decreased with increasing P application rates. Soil microbial biomass in the P-treated soils was significantly higher (P < 0.05) than those in the control. Shoot Zn concentration was positively correlated with Mehlich-3 extractable P (P < 0.0001) or exchangeable/carbonate-bound Zn (P < 0.001), but negatively related to soil pH (P < 0.0001). These results indicate that application of P fertilizers has the potential to enhance Zn mobility and uptake by hyperaccumulating plant S. alfredii, thus increasing phytoremediation efficiency of Zn-contaminated soils.

A nonpathogenic Fusarium oxysporum strain enhances phytoextraction of heavy metals by the hyperaccumulator Sedum alfredii Hance

Low biomass and shallow root systems limit the application of heavy metal phytoextraction by hyperaccumulators. Plant growth-promoting microbes may enhance hyperaccumulators'phytoextraction. A heavy metal-resistant fungus belonged to the Fusarium oxysporum complex was isolated from the Zn/Cd co-hyperaccumulator Sedum alfredii Hance grown in a Pb/Zn mined area. This Fusarium fungus was not pathogenic to plants but promoted host growth. Hydroponic experiments showed that 500 μM Zn(2+) or 50 μM Cd(2+) combined with the fungus increased root length, branches, and surface areas, enhanced nutrient uptake and chlorophyll synthesis, leading to more vigorous hyperaccumulators with greater root systems. Soil experiments showed that the fungus increased root and shoot biomass and S. alfredii-mediated heavy metal availabilities, uptake, translocation or concentrations, and thus increased phytoextraction of Zn (144% and 44%), Cd (139% and 55%), Pb (84% and 85%) and Cu (63% and 77%) from the original Pb/Zn mined soil and a multi-metal contaminated paddy soil. Together, the nonpathogenic Fusarium fungus was able to increase S. alfredii root systems and function, metal availability and accumulation, plant biomass, and thus phytoextraction efficiency. This study showed a great application potential for culturable indigenous fungi other than symbiotic mycorrhizas to enhance the phytoextraction by hyperaccumulators.